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Hard Rubber and Its Application in the Chemical Industries By A. C. Buttfield AMERICAN HARDRUBBERCo., BUTLER,N. J.
N PRACTICALLY every chemical plant there is some
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process or reaction where a corrosive reagent of one kind or another is used or produced. The selection of a suitable material for the containing and conveying of such a reagent is one of the important problems confronting the chemical engineer, for upon the proper selection may depend the success or failure of the entire enterprise. A great number of noncorrosive materials have, from time to time, been brought to the attention of the engineer, and as no single one of these will serve to solve every problem, it is necessary to carefully consider all before a final decision is reached. The material with which this paper is concerned and which, though used to some extent for many years, has recently aroused an increasing amount of interest, is hard rubber. NATUREOF HARD RUBBER As the average person has but a rather imperfect conception of the nature and composition of hard rubber, a brief description will, perhaps, be not out of place. Essentially, hard rubber is rubber compounded with a relatively high percentage of sulfur and cured for a comparatively long time at a relatively high temperature. While the line of demarcation between hard and soft rubber is not sharply defined, it is approximately correct to say that the sulfur content of hard rubber ranges from 15 to 50 per cent of the combined weight of the rubber and sulfur, the average being in the neighborhood of 30 per cent. Upon this factor-the ratio of rubber to sulfur-depends, to a large extent, the hardness and other properties of the final product. Of course it is possible to vary the hardness by limiting the cure, but in general this has been found to be bad practice, for incompletely vulcanized rubber is less stable than the properly cured product. Another means of changing the properties is by compounding the rubber with materials other than sulfur, and in this respect hard rubber resembles soft rubber; for just as pure rubber and sulfur, while excellent for such articles as elastic bands, would nevertheless be of but little value for automobile tires, so hard rubber containing nothing but rubber and sulfur, while excellent for making fountain pens, would be worthless for certain magneto parts. In this compounding experience and skill play an enormous part and upon the proper use of the right ingredients largely depends the value of the final product. Thus, it will be seen that the physical properties may be changed by varying the mixing and cure, and these changes, when properly controlled, make it possible to modify or increase the relative value of the resultant properties, so that the finished article may be most suited to the service intended. With this explanation in mind, it will readily be understood that it is impossible to set down fixed physical properties for hard rubber. The following will, however, serve as a guide : Tensile strength.. . . . . . . . . . . . 1500 t o 10,000 Ibs. per sq. in. Compressive strength.. . .. . 3000 t o 20 000 Ibs per sq. in Elongation, from 2 per cent in the harder, t'o 75 per cent in the softer grades Specific gravity, 1.14 upwards. Average about 1 2 0 Scleroscope hardness, 35 t o 65 Effect of heat-all but the specially hard grades soften noticeably a t 160' E.'.
For this reason hard-rubber chemical apparatus is not generally recommended for temperatures over 130' F.; not be-
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cause its resistance to corrosion is reduced, but rather because deformation is liable to take place. Hard rubber is nonporous, nonabsorbent, nonhygroscopic, and odorless. It may be machined, cut, threaded, sawed, ground; in fact, it resembles a metal in these respects. It is capable of taking a beautiful high polish; metal inserts may be molded in it and metal objects covered with it, Hard rubber has some remarkable electrical properties. Its dielectric strength runs as high as 1000 volts per mil, and its insulation resistance (resistivity) ranges up t o 12,000,000,000 megohms per centimeter. Tests made by the Bureau of Standards have shown a grade of hard rubber, prepared especially for radio work, to have a lower inductive capacity and phase angle a t radio frequencies than eleven other well-known materials. Hard rubber of suitable compound is chemically inert to a high degree and is successfully used in connection with a great many corrosive materials. Many laboratory tests have been made in which hard rubber has been immersed in various chemicals for long periods of time, and it would be possible to present many tables showing the effect of these chemicals in terms of loss or gain in weight per unit of surface. Such tables are often misleading, however, and it has been thought preferable to enumerate some typical materials in connection with which hard rubber has actually been commercially used for years. A few of these are: sulfuric acid of not over 1.50 specific gravity, nitric acid of not over 1.12 specific gravity, hydrochloric acid of any concentration; acetic, hydrofluoric, hydrofluosilicic, and phosphoric acids; sodium and potassium salts, such as hydroxides, chlorides, sulfates, etc.; zinc chloride, ferric chloride, stannic and stannous chlorides, calcium chloride, and many others, as well as corrosive gases, such as chlorine, hydrochloric acid gas, etc. Hard rubber is attacked by most rubber solvents, such as benzene, carbon disulfide, and chloroform, which tend to soften and swell it. EQUIPMENT OBTAINABLE
A very large variety of hard-rubber chemical equipment is obtainable. Pipe ranging in size from to 4 in. is regularly supplied, while larger sizes can be made. Fittings such as elbows, tees, unions, flanges, bibb cocks, valves, strainers, and ball floats are stocked, all with standard pipe threads, so that complete conveying lines can be readily assembled. Pumps, both reciprocating and centrifugal, in which all parts coming in contact with the solution to be circulated are either of solid hard rubber or metal entirely protected by seamless hard rubber, are made in sizes having capacities of from 6 to 122 gal. per min. A complete assortment of utensils, including buckets, measures, dippers, funnels, jars, baskets, scoops, etc., is available. Tanks of solid hard rubber as large as 15 ft. in length have been made, and such tanks can be supplied in almost any desired shape. Another adaptation of hard rubber is its use for lining or covering metal objects, such as iron tanks, centrifugal extractors, pipes, agitators, etc. By a recent discovery it has been found possible to improve upon the usual practice of placing the hard rubber in direct contact with the iron, by first applying a layer of speciaI soft-rubber compound, and then covering this with a layer of hard-rubber compound, the whole being vulcanized together. The soft rubber not only adheres to the iron much more tenaciously than will hard rubber, making separation of the tank and lining almost impossible, but also acts as a shock absorber and compensates for differences in expansion due fo temperature changes, thus protecting the hard rubber from possible fracture. In addition to the articles mentioned, much hard rubber in the form of sheet, rods, and tubes is used in various chemical
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
industries, as well as molded articles of many special sizes and shapes.
INDUSTRIES CSING HARD-RUBBER EQUIPMENT A complete list of industries in which hard-rubber equipment is used would be too long to fall within the scope of this paper. A few installations will be briefly described which will serve as a general guide for those who may be interested. Hard rubber is used perhaps most extensively for handling hydrochloric acid of various strengths. Installations of this kind include apparatus for conveying the acid from tank cars to storage tanks. This is done in three ways-first, by blowing with compressed air from the car through hardrubber distributing lines; second, by pumping, using hardrubber pumps and distributing through hard-rubber piping; third, by siphoning through hard-rubber pipes to hard-rubber air lifts, which raise the acid to tanks for distribution. Hard-rubber pumps and piping are also used in connection with hydrochloric acid for the extraction of rare metals from their ores; in the manufacture of galvanized iron wire, and like products; by dye manufacturers, who find hard rubber excellent for conveying acid through various steps in the manufacture of dyes. Special parts, such as dipping baskets, air-agitator systems, solid hard rubber and hard-rubber-lined tanks, are used extensively by metal-etching concerns. This equipment is also suitable for dilute nitric acid, ferric chloride, and other solutions used for etching purposes. Various manufacturers use thousands of feet of hard-rubber pipe for siphon systems on nitric acid absorption towers. These systems are used from the first tower in the series through to the point where the nitric acid concentration exceeds 16” BB. One of the largest single uses of hard-rubber equipment is in the silk-dyeing industry for handling stannic chloride solutions used in weighting silk. Typical installations include mixing tanks for building up the tin content of the spent tin solutions and soaking tanks where the silk is soaked in the solution until the required weight has been taken on. I n the so-called “dynamiting” process, the tin solution in a continuous system is pumped by hard-rubber pumps into a constant level hard-rubber-lined tank, which in turn feeds hard-rubber-lined and covered centrifugal extractors which are filled with silk for weighting purposes. The solution passes through the silk fibers and is discharged into a hardrubber-lined collecting tank, from which it is again pumped t o the constant-level tank. The liquid is fed to the extractors for a definite period of time, this time being calculated t o produce a certain amount of extra weight per pound of raw silk. Hard rubber is used in large quantities in the manufacture of chlorine and hypochlorites by the electrolytic process, as piping for distribution of the chlorine gas from the cells and for collecting the hypochlorite liquor. Concerns engaged i n the manufacture of chlorine often furnish hard-rubber distributing lines with their chlorine containers. These lines convey the gas direct to bleaching processes in various industries, many of which use agitating systems forcing the gas through hard-rubber pipes drilled with small holes and immersed in tanks containing the material to be bleached. Chlorine control apparatus for use with flour and cottonbleaching processes is often of hard-rubber construction. Here hard-rubber towers, float balls, and various forms of special equipment are indispensable. An interesting development in the use of hard rubber is shown in the handling of plating solutions, many of which have a tendency not only to corrode, but also to form deposits on the surfaces of metallic containers. Hard-rubber goosenecks, electrode separators, and tanks are used in gen-
eral plating work, the tanks serving to reduce electrical leakage to a minimum, owing not only to the high insulating properties of hard rubber, but also because there are no pores in which the solution may crystallize as it does, for instance, in the cellular surfaces of wooden tanks. Many installations of hard-rubber equipment have been made in plants devoted to the manufacture of food products, where its resistance to corrosion and its nonporous and nonabsorbent properties make it ideal for handling such material as cider, fruit juices, vinegar, etc. Cider and vinegar plants use hard rubber very extensively, not only as pumps, piping, and fittings, but also in the form of special filtering apparatus, tilting vessels, and special distributors or sparges which are used in every vinegar generator. Condiment manufacturers use hard-rubber pipe, fittings, tanks, and special apparatus throughout their mixing and other processes, and even through to the point where the condiments are bottled. Very interesting results have been obtained recently in connection with experiments in the transportation of hydrochloric acid by means of hard-rubber-lined iron tanks mounted on tank cars. These experiments, while not yet altogether successful, show promise; and it seems probable that they may be carried on to a successful conclusion. Many other uses of hard-rubber equipment might be described, but those mentioned will serve to give a comprehensive idea of the wide range of usefulness of this unique material, and to show that it is worthy of consideration wherever the handling of corrosive substances prevents the use of the more common materials of chemical-equipment construction.
New H a v e n Registration The registration, by states, of those present at the New Haven meeting was as follows : 4 Oklahoma 2 Louisiana Arkansas 7 Maine 1 Omaha California 18 Pennsylvania Colorado 2 Maryland 108 Rhode Island 146 Massachusetts ConnecticLI t 25 Michigan 14 Texas Delaware 5 Vermont District of C,olumbia 6 3 Minnesota 7 Virginia 2 Missouri Georgia 1 Washington Illinois 38 Nebraska 13 West Virginia 7 New Jersey Indiana 265 Wisconsin 1 New York Iowa 4 Foreign 4 North Carolina Kansas 1 Ohio 56 Kentucky Guests 218 TOTAL REGISTRATION 1151
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1 2 72 13 5
7 7 1 8 8
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Committee on Standardization of C u p r a m m o n i u m M e t h o d for Viscosity of Cellulose The AMERICANCHEMICALSOCIETY, through its Cellulose Division, has appointed a committee to standardize the determination of the viscosity of cuprammonium cellulose solutions as a control method. This committee is desirous of obtaining the cooperation of all individuals or firms who are interested in the cuprarnmonium method for viscosity of cellulose. ‘It is generally agreed that the method is of considerable importance in the cellulose industry, and no doubt there is considerable information available from widely scattered sources. The committee is anxious to obtain the assistance of anyone who is willing to cooperate o n this matter. Descriptions of methods now in use, together with comments as to possible means of improvement, or general discussion of the method and the results obtainable by it, will be welcomed by any member of the committee. The members of this committee are: I,. 0. LITTLETON, Massasoit Mfg. Co., Fall River, Mass. E. F. J. PUCKHABER,Stamsocott Co., Hopewell, Va. C. S. VENABLE,Viscose Co., Marcus Hook, Pa. W. 0. MITSCHERLING, 2018 Jefferson St., Wilmington, Del
